Teladorsagia circumcincta (previously known as Ostertagia circumcincta) is the major cause of parasitic gastroenteritis in small ruminants in temperate regions. This nematode is controlled primarily by anthelmintics; however resistance is widespread and field isolates have often been found to be insensitive to a number of different anthelmintic classes (Bartley et al., 2004; Wrigley et al., 2006). T. circumcincta resides within the abomasum (or true stomach) of small ruminants and primarily causes disease in animals during their first year of grazing. It is a major cause of production losses, estimated to cost the UK sheep industry alone in excess of £80 M per annum (Nieuwhof & Bishop, 2005). The associated clinical signs range from suppressed appetite to diarrhoea, dehydration and death; however, the major impact of teladorsagiosis is its effect on lamb productivity via a reduction in weight gain (Gibson and Everett, 1976).
Protective immunity against challenge with T. circumcincta develops after continual (‘trickle’) infection over a number of weeks (Seaton et al., 1989). The degree of immunity that develops depends on a number of factors including, level of parasite challenge, age of animal and its genotype (Singleton et al., 2011). In ewes that have acquired immunity to T. circumcincta, resistance to the parasites can lapse around the time of parturition and early lactation (Houdijk et al., 2005). In terms of anti-parasite effects, the protective immune response has been shown to decrease the establishment of larvae in the abomasal mucosa, slow larval development in the gastric gland and to reduce the egg output of female worms in the abomasal lumen (Balic et al., 2003; Seaton et al., 1989; Smith et al., 1985, 1986; Stear et al., 2004). Experiments that demonstrated successful adoptive transfer between immune and naive sheep using gastric lymph indicate the importance of local immune responses in protective mechanisms against T. circumcincta (Smith et al., 1986). The precise mechanisms remain to be defined, but roles for both immediate hypersensitivity reactions and local antigen specific IgA have been highlighted (Smith et al., 1986; 1987). Furthermore, antigen-specific IgA responses have been correlated with reductions in nematode length (Halliday et al., 2007; Smith et al. 2009), whereas IgE responses have been correlated with a reduction in faecal egg counts in grazing lambs (Huntley et al., 2001).
As sheep can acquire a protective immune response against T. circumcincta in natural and experimental circumstances, vaccination represents a possible alternative for control.